Crankshaft and method for manufacturing same

ABSTRACT

A surface of a steel, as a material for a crankshaft, is nitrocarburized. The steel contains, as alloy elements C having a content 0.10 mass % or more 0.30 mass % or less, Si having a content 0.5 mass % or more and 0.3 mass % or less, Mn having a content 0.3 mass % or more and 1.5 mass % or less, Mo having a content 0.8 mass % or more and 2.0 mass % or less, Cr having a content 0.1 mass % or more and 1.0 mass % or less, and V having a content 0.1 mass % or more and 0.5 mass % or less, with a remainder consisting of Fe and inevitable impurities. The contents of the alloy elements fall within ranges: 2.0 mass %≦Mn+Cr+Mo≦3.0 mass %, 2.3 mass %≦C+Mo+5V ≦3.7 mass %, and 2.7 mass %≦2.16 Cr+Mo+2.54V≦4.0 mass %. If a steel sample extracted from a central portion of the nitrocarburized steel free from an influence of the nitrocarburizing treatment is austenitized at 1200° C. for one hour, and cooled to a room temperature so that a cooling rate at which the steel sample passes through a temperature range between 900° C. and 300° C. is 0.5° C./second, then an area percentage of a bainite structure in steel structures is 80% or more and a Vickers hardness measured at a cross section is 260 Hv or more and 330 Hv or less. A surface hardness of a nitrocarburized layer is 650 Hv or more, a formation depth of the nitrocarburized layer is 0.3 mm or more, and a hardness of the central portion is 340 Hv or more. Thereby a crankshaft which is excellent both in the machinability and in fatigue strength, even after nitrocarburizing treatment on the surface, is provided.

RELATED APPLICATIONS

This application claims the priority of Japanese Patent Application No. 2005-115112 filed on Apr. 12, 2005, which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a crankshaft consisting of a steel that includes a nitrocarburized layer on a surface, and a method for manufacturing the crankshaft.

2. Description of the Related Art

Patent Literature 1: Japanese Patent Application Laid-Open No. 10-030632

Patent Literature 2: Japanese Patent Application Laid-Open No. 06-128690

Patent Literature 3: Japanese Patent Application Laid-Open No. 05-279795

Patent Literature 4: Japanese Patent Application Laid-Open No. 05-279794

An automobile crankshaft is used in an environment in which a high torsional load and a high bending load repeatedly act on the crankshaft. The crankshaft is, therefore, required to be excellent in static strength and fatigue strength. On the other hand, since the crankshaft is a member quite large in size and complicated in shape, it is normally and basically manufactured using a non-heat treated steel that is not quenched and tempered after being hot-forged. In this case, to ensure strength, it is necessary to finally perform a hardening treatment on a surface of the steel. The Patent Literatures 1 to 4 disclose methods using a nitrocarburizing treatment as the surface hardening treatment. The nitrocarburizing treatment is a treatment in which a workpiece is treated in, for example, an ammonia gas atmosphere at a temperature equal to or lower than the A1 transformation point or generally at a temperature of about 570 degrees Centigrade, part of carbon as well as nitrogen is introduced into the steel, nitrides or carbides are produced, and a surface layer of the steel is thereby hardened. Such a nitrocarburizing treatment is suited for mass-production of crankshafts that are large-sized engine parts of an automobile since the treatment hardly generates strains in the workpiece differently from a carburizing and quenching method, and does not require a long time differently from a nitriding method.

Meanwhile, by performing the nitrocarburizing treatment, it is expected that a hardness of the surface layer of the crankshaft is greatly increased by introduction of the nitride thereinto. However, it is difficult to diffuse nitride into the steel at a depth equal to or larger than 0.5 millimeters, and an interior of the crankshaft cannot be reinforced by a nitrocarburizing treatment normally performed for a few hours. On the contrary and normally, in the nitrocarburizing treatment, since the heat of the steel is held at about 600 degrees Centigrade, the interior into which the nitrogen is not introduced is softened by a thermal history of the nitrocarburizing and the hardness is rather lower than that before the treatment.

On the other hand, if the internal strength of the steel before the nitrocarburizing treatment is to be increased by addition of components to the steel or the like, then machinability of the steel is greatly deteriorated. As a result, efficiency for machining the steel into a shape of the crankshaft before the nitrocarburizing treatment is deteriorated, making it difficult to industrially produce parts. Further, the hardness of the steel before the nitrocarburizing treatment can be increased by quenching the steel after machining. However, the quenching treatment deforms the part and requires a step of removing scales after the quenching treatment and the like. Thus, the quenching treatment degrades a quality and considerably increases a machining cost.

Moreover, an ordinary crankshaft is complicated in shape and it is essential to cut the steel after hot forging. Conventionally, at a cutting step after a normalizing treatment, chips generated are wound around a product or a wear resistance of a tool is deteriorated by the chips. To prevent these, Pb is normally contained in the steel as a chip crushability improving element. However, use of Pb is gradually avoided and being restricted recently since an environmental preservation attracts increasing attention on the global scale.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a crankshaft capable of ensuring both excellent machinability and high fatigue strength despite a nitrocarburizing treatment performed on a surface thereof, and a method for manufacturing the crankshaft.

To attain this object, according to a first aspect of the present invention, there is provided a crankshaft consisting of a steel having a surface subjected to a nitrocarburizing treatment, characterized by comprising:

a pin; and

a journal, wherein

the steel contains, as alloy elements:

C having a content equal to or more than 0.10 mass % and equal to or less than 0.30 mass %,

Si having a content equal to or more than 0.05 mass % and equal to or less than 0.3 mass %,

Mn having a content equal to or more than 0.3 mass % and equal to or less than 1.5 mass %,

Mo having a content equal to or more than 0.8 mass % and equal to or less than 2.0 mass %,

Cr having a content equal to or more than 0.1 mass % and equal to or less than 1.0 mass %, and

V having a content equal to or more than 0.1 mass % and equal to or less than 0.5 mass %, with a remainder consisting of Fe and inevitable impurities;

the contents of the alloy elements fall within the following ranges:

2.3 mass %≦C+Mo+5V≦3.7 mass %,

2.0 mass %≦Mn+Cr+Mo≦3.0 mass %, and

2.7 mass %≦2.16 Cr+Mo+2.54V≦4.0 mass %;

if a steel sample extracted from a central portion of the nitrocarburized steel free from an influence of the nitrocarburizing treatment is austenitized at 1200 degrees Centigrade for one hour, and cooled to a room temperature so that a cooling rate at which the steel sample passes through a temperature range between 900 and 300 degrees Centigrade is 0.5° C./second, then an area percentage of a bainite structure in steel structures is equal to or higher than 80 percent and a Vickers hardness measured at a cross section is equal to or higher than 260 Hv and equal to or lower than 330 Hv, and

The pin and the journal have a surface hardness of a nitrocarburized layer is equal to or higher than 650 Hv, a formation depth of the nitrocarburized layer is equal to or larger than 0.3 millimeters, and a hardness of the central portion is equal to or higher than 340 Hv.

Further, according to a second aspect of the present invention, there is provided a method for manufacturing the crankshaft according to the first aspect of the present invention, characterized in that

after the steel is hot—forged or hot—forged and subjected to a solution treatment to have a shape including the pin and the journal at a temperature equal to or higher than 900 degrees Centigrade and lower than a melting point of the steel, the steel is cooled so that the cooling rate of the pin and the journal is equal to or higher than 0.3° C./second and equal to or lower than 2° C./second, whereby the percentage of the bainite structure in the steel structures is equal to or higher than 80 percent, and

thereafter, the pin and the journal are subjected to a cutting then the surface of the steel is subjected to a nitrocarburizing treatment.

The crankshaft can be constituted to have a structure in which crank arms arranged at predetermined intervals in a direction of a rotation axis are alternately coupled by crank journals arranged so that a central axis of the journals coincides with the rotation axis and crank pins each having a central axis at a location away from the rotation axis by a certain distance in a radial direction.

The crankshaft and the crankshaft manufacturing method according to the present invention can reduce the hardness of the material before the nitrocarburizing treatment and ensure excellent machinability. In addition, they can attain a high surface hardness by the nitrocarburizing treatment and harden the interior of the steel by heat-holding during the nitrocarburizing. Therefore, the crankshaft and the crankshaft manufacturing method according to the present invention can also ensure excellent fatigue strength besides the excellent machinability.

The crankshaft according to the present invention uses a steel composition that can improve hardness of the interior of the steel by heat-holding during the nitrocarburizing. The steel contains, as essential alloy elements, Mo, V, and Ti. Those elements are known as carbide generators. In the steel as a constituent material of the crankshaft according to the present invention, addition amounts of these elements are adjusted so that the elements function not only as carbide generators but also control elements that control structures of a steel matrix or specifically control the bainite structure if the steel is air-cooled after being hot-forged to obtain the shape of the crankshaft.

By adding the Mo, V, and Ti, if the steel is held at near the nitrocarburizing treatment temperature, secondary precipitation strengthening can be expected by the carbides generated by these elements. However, if a material state before the nitrocarburizing treatment is turned into a state of a martensitic structure generated by the quenching and tempering, the hardness of the steel in material state is excessively increased. This deteriorates the machinability and often causes problems due to quenching strains. In addition, the hardness improving effect by the carbide precipitation is achieved less conspicuously and the significance of adding the expensive carbide precipitation elements becomes unclear. According to the present invention, the Mo, V, and Ti contents are appropriately adjusted, whereby the material state before the nitrocarburizing is made to be a state of a bainite structure. As compared with the martensitic structure, the bainite structure is low in hardness and it is relatively easy to perform a machining processing including a cutting processing on the bainite structure. As a result, it is possible to sufficiently increase the internal hardness by the nitrocarburizing treatment and improve the fatigue strength while ensuring the same excellent machinability as the pre-nitrocarburized one. Furthermore, by turning the material state into the bainite structure, it is possible to suppress cutting chips becoming continuously connected, and effectively suppress defects such as winding of the chips around the cutting tool at the time of cutting the steel into the shape of the crankshaft.

A shape stock of the crankshaft is manufactured by the hot forging or the like. The cooling rate after the hot forging varies depending on dimensions and the shape of the crankshaft. It is preferable to cool the steel so that the bainite structure can be obtained in a wide cooling range. The steel structures of the steel used for the crankshaft according to the present invention are adjusted so that the area percentage of the bainite structure in the steel structures before the nitrocarburizing treatment is equal to or higher than 80 percent, and so that the hardness is equal to or higher than 260 Hv and equal to or lower than 330 Hv. However, in the crankshaft that has been finally nitrocarburized, the structures before the nitrocarburizing treatment cannot be directly specified. On the other hand, as a result of calculating the cooling rate of an actual part, it is confirmed that an average cooling rate of the pin and the journal, when the material is cooled after being hot-forged, falls within the range from 0.3° C./second and 2° C./second that are required to have sufficient strength. Therefore, a steel sample is extracted from a central portion of the nitrocarburized steel free from an influence of the nitrocarburizing treatment is austenitized at 1200 degrees Centigrade for one hour, and cooled so that the cooling rate at which the steel sample passes through a temperature range between 900 and 300 degrees Centigrade is 0.5° C./second. By doing so, as long as the area percentage of the bainite structure in the steel structures is equal to or higher than 80 percent and the Vickers hardness measured at a cross section is equal to or higher than 260 Hv and equal to or lower than 330 Hv, it is possible to attain the percentage of the bainite structure and the hardness substantially equal in level to those before the nitrocarburizing treatment in the crankshaft manufacturing steps including a step of cooling the steel after hot-forging the steel.

Reasons for limiting the steel structures and numerical parameters adopted in the present invention will now be described.

“C having a content equal to or more than 0.10 mass % and equal to or less than 0.30 mass %.”

The C is a necessary element to secure the strength. However, if the C content is less than 0.10 mass %, the strength cannot be secured. If the C content exceeds 0.30 mass %, the hardness of the material before cutting (the material after a hot forging treatment, a normalizing treatment, or a solution treatment) is excessively increased. This deteriorates cutting machinability.

“Si having a content equal to or more than 0.05 mass % and equal to or less than 0.3 mass %”

The Si is contained in the steel as an element serving as a deoxidizer during smelting steel and serving to improve the fatigue strength. If the Si content is less than 0.05 mass %, desired effects cannot be achieved. If a large amount of the Si is added so that the Si content exceeds 0.30 mass %, the degree of nitrocarburizing the steel is reduced. As a result, a predetermined surface hardness cannot be attained.

“Mn having a content equal to or more than 0.3 mass % and equal to or less than 1.5 mass %”

The Mn is an element that plays an important role in the present invention and that is essential to generate the bainite structure in the material structures before the nitrocarburizing treatment. Specifically, the Mn content is adjusted according to the Cr and Mo contents. If the Mn content is less than 0.3 mass %, the generation of the bainite structure is unstable. Therefore, it is necessary to add the Mn at the content equal to or higher than 0.3 mass %. If a large amount of the Mn is added so that the Mn content exceeds 1.5 mass %, the hardness is increased but, at the same time, an increase in the internal hardness after the nitrocarburizing treatment cannot be expected.

“Mo having a content equal to or more than 0.8 mass % and equal to or less than 2.0 mass %”.

“V having a content equal to or more than 0.1 mass % and equal to or less than 0.5 mass %”.

The Mo and V are elements, that play important roles in the present invention, and that are added so as to increase the internal hardness by heating during the nitrocarburizing treatment. The Mo functions to make the internal hardness higher along with the increase of the addition amount of the Mo. If the Mo content is equal to or lower than 0.8 mass %, the effect of increasing the hardness cannot be achieved as intended. If the Mo is added so that the Mo content exceeds 2.0 mass %, the increase in the internal hardness can be attained. However, a hardness of the bainite in the material state is excessively high, thereby deteriorating the machinability. The V functions to precipitate V carbides and increase the material hardness by air cooling after the hot forging. In addition, the V functions to make the internal hardness higher along with the increase of the addition amount of the V. If the V content is less than 0.1 mass %, the effect of increasing the hardness cannot be achieved as intended. If the V is added so that the V content exceeds 0.5 mass %, the increase in the internal hardness can be attained. However, the hardness of the bainite in the material state is excessively high, thereby deteriorating the machinability.

“Cr having a content equal to or more than 0.1 mass % and equal to or less than 1.0 mass %”

The Cr is also an element that plays an important role in the present invention. The Cr is added so as to stabilize the surface hardness after the nitrocarburizing treatment and to attain the hardness equal to or higher than 650 Hv. If the Cr content is less than 0.1 mass %, the hardness is insufficiently low after the nitrocarburizing treatment. If the Cr content exceeds 1.0 mass %, the effect saturates.

2.0 mass %≦Mn+Cr+Mo≦3.0 mass %,

“2.3 mass %≦C+Mo+5V≦3.7 mass %, and

2.7 mass %≦2.16 Cr+Mo+2.54V≦4.0 mass %”.

The present invention is characterized by increasing the internal hardness by heat-holding during the nitrocarburizing. However, the effect of increasing the internal hardness cannot be sufficiently achieved even if the Mo and V (also additionally Ti and Nb as arbitrarily added elements to be described later) are simply added as carbide generators. The secondary precipitation of carbides is a phenomenon that can be recognized in a temperature range around 600 degrees Centigrade, and is normally used in the quenching and tempering. However, if the material state is turned into that of the martensitic structure as done in the quenching and tempering, it is difficult to increase the internal hardness by the carbide precipitation, and the hardness higher than that obtained by the quenching cannot be obtained.

To solve this disadvantage, according to the present invention, the material state is made to be the state of the bainite structure. In this material state, the hardness is low. The steel is heat-held at the temperature range about 600 degrees Centigrade so as to make it possible to increase the internal hardness and to make the hardness higher than that in the material state. The inventors of the present invention considered addition amounts of the alloy elements so as to stably obtain the bainite structure by conducting various investigations. As a result, the inventors discovered that the contents of the alloy elements of Mn, Cr, and Mo preferably satisfy the relationship represented by 2.0 mass %≦Mn+Cr+Mo ≦3.0 mass %. The shape stock of the crankshaft is manufactured by the hot forging or the like. The cooling rate after the hot forging varies according to the dimensions and shape of the crankshaft. It is preferable to cool the steel so that the bainite structure can be obtained in a wide cooling range. As stated, the average cooling rate of the pin and the journal when the material is cooled after being hot-forged into the shape of the crankshaft falls within the range from 0.3 ° C./second and 2° C./second that are required to have sufficient strength.

As a result of further dedicated studies, the inventors of the present invention discovered as follows. If a sum of the Mn, Cr, and Mo contents is set to be less than 2.0 mass % (2.0 mass %>Mn+Cr+Mo) to obtain the bainite structure in the process of the hot forging, then the area percentage of the generated bainite structure is not higher than 80%, and the effect of increasing the hardness by the heat-holding cannot be expected. On the other hand, the generation of the bainite structure relates to the C content. If the sum of the Mn, Cr, and Mo contents is higher than 3.0 mass % (Mn+Cr+Mo>3.0 mass %), the hardness of the material is excessively increased, thereby deteriorating the machinability.

Furthermore, in order to appropriately increase the hardness by the heat-holding, it is necessary to satisfy the relationship represented by 2.3 mass %≦C+Mo+5V≦3.7 mass %. At 2.3 mass %>C+Mo+5V, an allowance for the increased amount of the hardness by the heat-holding is insufficient. At C+Mo+5V>3.7 mass %, the hardness of the material is excessively increased, thereby deteriorating the machinability. In addition, in order to appropriately increase the surface hardness by nitrocarburizing, it is necessary to satisfy the relationship represented by 2.7 mass %≦2.16 Cr+Mo+2.54V≦4.0 mass %. At 2.7 mass %>2.16 Cr+Mo+2.54V, the allowance for the increased amount of the hardness by the nitrocarburizing is insufficient. At 2.16 Cr+Mo+2.54V>4.0 mass %, the hardness of the material is excessively increased, thereby deteriorating the machinability.

The area percentage of the bainite structure in the steel structures is set equal to or higher than 80 percent and the Vickers hardness measured at a cross section is set equal to or higher than 260 Hv and equal to or lower than 330 Hv for the following reasons. As already stated, it is necessary to turn the structural state before the nitrocarburizing into that of the bainite structure so as to increase the internal hardness simultaneously with the nitrocarburizing treatment. The reason is as follows. If the steel is heated holding the temperature range about 600 degrees Centigrade regardless the structure shape before the nitrocarburizing, carbides are precipitated. However, to conspicuously increase the hardness by the precipitation strengthening, it is necessary to turn the structure of the material state into the bainite structure. In addition, after the structure subjected to the hot forging is softened by annealing or the like and machined, the steel is reheated and re-cooled, whereby the structure can be adjusted and the hardness can be adjusted. However, the addition of another heat treatment causes a cost increase. Besides, oxidized scales are generated by heating and cooling. Therefore, it is preferable to adjust the structure to a predetermined structure and adjust the hardness to a predetermined hardness while the steel is being hot-forged. In this case, the steel is machined in a state after the hot forging. It is, therefore, necessary to ensure the machinability. In light of the balance between the machinability and the strength, the Vickers hardness is set to be equal to or higher than 260 Hv and equal to or lower than 330 Hv.

The surface hardness after the nitrocarburizing treatment is set to be equal to or higher than 650 Hv, a depth of an entire hardened layer is set to be equal to or larger than 0.3 millimeters, and the hardness of the central portion is set to be equal to or higher than 330 Hv for the following reasons. Since a bending stress and a torsional stress repeatedly act on the crankshaft, the crankshaft needs to have a high bending fatigue strength and a high torsional fatigue strength. In each of the fatigue phenomena, a maximum load stress acts on an uppermost surface of the steel. It is, therefore, important to increase the hardness of the surface layer so as to improve the fatigue strength. The higher hardness of the surface layer is more advantageous. It is confirmed that the surface hardness is increased by adding Cr, Al and the like in the nitrocarburizing treatment. If these elements are added, the depth of the hardened layer tends to be smaller. The inventors of the present invention performed a strength evaluation using an actual part. As a result, it was confirmed that the strength is reduced even if the hardness of the surface layer is increased as long as the depth of the hardened layer is small. The inventors of the present invention conducted dedicated studies of the balance between the optimum hardness of the surface layer and the optimum depth of the hardened layer so as to ensure sufficiently high fatigue strengths. As a result, the inventors discovered that it is preferable to set the surface layer hardness to equal to or higher than 650 Hv (and equal to or lower than, for example, 950 Hv), the depth of the nitrocarburized (hardened) layer to be equal to or higher than 0.3 millimeters (and equal to or lower than, for example, 1.5 millimeters), and to set the hardness of the central portion to be equal to or higher than 330 Hv (and equal to or lower than, for example, 430 Hv).

The steel used as the material for the crankshaft according to the present invention can further contain the following elements.

“Nb having a content equal to or more than 0.02 mass % and equal to or less than 0.2 mass %.”

“Ti having a content equal to or more than 0.005 mass % and equal to or less than 0.2 mass %.”

Similarly to the Mo, the Nb and Ti function to precipitate carbides by the heat-holding during the nitrocarburizing and to thereby increase the internal hardness. Therefore, the Nb and Ti are added to the steel if it is necessary to do so. However, if the Nb and Ti are added so that the Nb and Ti contents exceed their respective upper limits, large-sized crystallized substances are generated at a forging step in manufacturing the steel by an ordinary method. As a result, effective Nb and Ti that contribute to the increase in the internal hardness cannot be obtained. It is, therefore, preferable to set each of the upper limits of the Nb and Ti contents to 0.2 mass %. “Al having a content equal to or more than 0.003 mass % and equal to or less than 0.1 mass %.”

The Al can be added so as to increase the surface hardness. However, if the Al content is less than 0.003 mass %, the effect of increasing the surface hardness cannot be conspicuously achieved. If the Al content is increased, the surface hardness is increased proportionally. However, if the Al is excessively added, the diffusion of nitrogen into the steel during the nitrocarburizing is obstructed and the hardened layer is made shallower. It is, therefore, preferable to set the upper limit of the Al content to 0.1 mass % so that the Al content can be prevented from adversely influencing the depth of the hardened layer and so that the Al can be expected to function only to increase the surface hardness.

“S having a content equal to or more than 0.01 mass % and equal to or less than 0.1 mass %.”

“Ca having a content equal to or more than 0.0010 mass % and equal to or less than 0.010 mass %.”

The S and Ca are elements used to improve the machinability in the machining of the steel. By dispersing MnS, Ca oxide, and Ca sulfide into the steel, the machinability is improved. If the S and Ca contents are less than their respective lower limits, the effect of improving the machinability cannot be conspicuously achieved. If they exceed their respective upper limits, a toughness of the steel is deteriorated.

The steel used as the material for the crankshaft according to the present invention may contain elements other than the above-stated essential components such as Cu, Ni, P, and O within the range in which the effects of the present invention are not reduced. The Cu and Ni, contents of which are about 0.10 mass %, may possibly mixed into the steel as inevitable impurities from scraps or the like. The P and O are elements that may possibly be mixed into the steel as inevitable impurities produced at a steel manufacturing process. Since the P deteriorates the toughness of the steel, a P content is preferably set to be equal to or less than 0.0030 mass %.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of one example of a crankshaft according to the present invention.

DESCRIPTION OF THE PRESENT INVENTION

FIG. 1 is a front view of one example of a crankshaft according to the present invention. The crankshaft 1 is configured so that crank arms 2 arranged at predetermined intervals in a direction of a rotational axis O are alternately coupled by a crank journal 4 arranged so that a central axis of the journal 4 coincides with the rotational axis O, and by crank pins 5 each having a central axis at a position away from the rotational axis O by a certain distance. A hole 8 for injecting oil is formed in each crank pin 5. The two crank arms 2 form a proximal surface formation portion in which a surface of each crank arm 2 whose surface faces the adjacent crank arm 2 is a flat proximal surface 2 a. A fillet 7 an outside diameter of which is gradually larger as closer to the proximal surface 2 a side, is formed on a protruding proximal end of the crank journal 4 and the crank pin 5 (an axis-like portion). The protruding proximal end is concave, so that a stress tends to concentrate thereon. However, by forming the fillet 7, the concentration of the stress is relaxed and a bending strength of the crankshaft 1 can be increased.

Each of the crank journal 4 and the crank pin 5 is formed into an axis having a circular cross section. After the steel having the composition stated above is hot-forged, a nitrocarburized layer is formed on an entire outer circumference of the steel. The crankshaft 1 thus configured is manufactured as follows. Materials are molten, cast, and divided into blocks so as to obtain the steel having the composition already described above in detail. Thereafter, the divided steel block is hot-forged and then air-cooled. By air cooling, a cooling rate at the crank journal 4 and the crank pin 5 is within the range between 0.3° C./second and 2° C./second. By adopting the composition, in the steel before the nitrocarburizing, an area percentage of a bainite in structures before nitrocarburizing is equal to or higher than 80 percent, and a hardness of the steel is equal to or higher than260Hv and equal to lower than 330 Hv. Since the structures mainly consist of the bainite structure, the steel can be easily machined into a crankshaft shape by cutting. After the cutting, the resultant member is nitrocarburized in an ammonia gas atmosphere. The nitrocarburizing treatment is performed at a temperature equal to or higher than 550 degrees Centigrade and equal to or lower than 700 degrees Centigrade (e.g., at 600 degrees Centigrade). Since an interior of the steel is the bainite structure, fine carbides are conspicuously precipitated during the nitrocarburizing treatment differently from a quenched structure consisting of a martensitic structure, thereby improving a strength of the interior of the steel. A surface hardness of the nitrocarburized steel is set equal to or higher than 650 Hv, a depth of an entire hardened layer is set equal to or larger than 0.3 millimeters, and a hardness of a central portion of the steel is set equal to or higher than 340 Hv. Thereafter, a well-known cold straightening treatment is performed on the steel using a straightening roll or the like so as to correct deformations, strains, and the like of the steel generated by the nitrocarburizing treatment.

EXAMPLE

A result of an experiment conducted to confirm the effects of the present invention will be described.

A steel having a chemical composition shown in Table 1 below is molten in a five-ton arc furnace or a 150-kilogram high-frequency vacuum induction furnace. A resultant steel ingot is rolled or forged into a round rod having a diameter of 90 millimeters. TABLE 1 Chemical components (main) Chemical components (sub) Component formula upper limit 0.30 0.30 1.50 2.00 1.00 0.50 0.20 0.20 0.100 0.10 0.0100 3.70 3.00 4.00 lower limit 0.10 0.05 0.30 0.80 0.10 0.10 0.02 0.01 0.003 0.01 0.0010 2.30 2.00 2.70 C Si Mn Mo Cr V Nb Tl Al S Ca {circle around (1)} {circle around (2)} {circle around (3)} invention 1 0.12 0.24 0.60 1.50 0.35 0.28 0.05 0.04 3.02 2.45 2.97 2 0.25 0.25 0.81 1.30 0.40 0.30 0.03 0.05 3.05 2.51 2.93 3 0.16 0.12 0.91 1.50 0.40 0.17 0.04 0.06 2.51 2.81 2.80 4 0.16 0.25 0.84 1.60 0.33 0.18 0.04 0.05 2.66 2.77 2.77 5 0.17 0.26 0.40 1.70 0.34 0.22 0.02 0.04 2.97 2.44 2.99 6 0.18 0.26 1.30 1.31 0.35 0.30 0.05 0.04 2.99 2.96 2.83 7 0.18 0.27 0.87 0.90 0.56 0.33 0.07 0.05 2.73 2.33 2.95 8 0.20 0.25 0.69 1.82 0.45 0.30 0.04 0.06 3.52 2.96 3.55 9 0.19 0.29 0.60 1.28 0.20 0.41 0.03 0.05 3.52 2.08 2.75 10 0.20 0.25 0.74 1.43 0.81 0.20 0.04 0.06 2.63 2.98 3.69 11 0.19 0.24 0.75 1.29 0.45 0.18 0.05 0.04 2.38 2.48 2.72 12 0.15 0.25 0.70 1.30 0.67 0.40 0.05 0.04 3.45 2.67 3.76 13 0.19 0.24 0.75 1.30 0.50 0.18 0.05 0.07 0.04 2.39 2.55 2.84 14 0.19 0.29 0.75 1.31 0.56 0.18 0.15 0.02 0.06 2.45 2.62 3.00 15 0.16 0.25 0.94 1.33 0.65 0.20 0.06 0.03 0.04 2.49 2.92 3.24 16 0.19 0.26 0.75 1.32 0.50 0.20 0.16 0.04 0.05 2.51 2.57 2.91 17 0.18 0.25 0.83 0.91 0.46 0.30 0.05 0.04 2.59 2.20 2.67 18 0.19 0.24 0.75 1.24 0.48 0.24 0.07 0.06 2.63 2.47 2.89 19 0.20 0.24 0.76 0.99 0.61 0.25 0.02 0.04 2.44 2.36 2.94 20 0.21 0.23 0.75 1.33 0.71 0.20 0.03 0.05 2.54 2.79 3.37 21 0.19 0.24 0.75 1.30 0.41 0.18 0.04 0.04 0.0035 2.38 2.46 2.64 comparison 1 0.05 0.25 0.80 0.90 0.60 0.20 0.03 0.04 1.95 2.30 2.70 2 0.35 0.15 1.00 1.60 0.50 0.20 0.04 0.05 2.95 3.10 3.19 3 0.21 0.03 0.70 1.20 0.70 0.20 0.03 0.06 2.41 2.60 3.22 4 0.19 0.34 0.80 1.43 0.50 0.20 0.03 0.05 2.62 2.73 3.02 5 0.20 0.25 0.18 1.40 0.40 0.20 0.05 0.04 2.60 1.98 2.77 6 0.22 0.24 2.00 1.30 0.50 0.30 0.05 0.05 3.02 3.60 3.14 7 0.23 0.25 0.50 0.60 0.60 0.20 0.03 0.06 1.83 1.70 2.40 8 0.21 0.24 0.60 2.60 0.60 0.20 0.04 0.05 3.81 3.80 4.40 9 0.18 0.24 0.70 1.30 0.05 0.20 0.03 0.06 2.48 2.05 1.92 10 0.18 0.25 0.50 1.26 1.30 0.20 0.04 0.04 2.47 3.08 4.60 11 0.20 0.26 0.65 1.30 0.50 0.05 0.03 0.04 1.75 2.45 2.51 12 0.21 0.24 0.72 0.75 0.55 0.70 0.04 0.04 4.46 2.02 3.72 13 0.24 0.25 0.70 1.43 0.62 0.40 0.40 0.04 0.06 3.67 2.75 3.79 14 0.25 0.25 0.70 1.50 0.60 0.20 0.30 0.03 0.03 2.75 2.80 3.30 hardness + structure upper limit 330HV — 430HV — — — lower limit 260HV 650HV 340HV 80% 560 MPa 100 hardness surface hardness central portion after after hardness after bainite area fatigue machinability forging nitrocarburizing nitrocarburizing percentage strength index invention 1 272 674 340 81 573 129 2 308 670 391 96 642 106 3 293 656 351 90 589 114 4 290 653 352 89 590 116 5 276 677 343 84 577 126 6 317 659 405 100 658 102 7 290 672 354 87 593 116 8 317 737 427 102 684 102 9 277 651 362 88 605 125 10 319 752 392 100 643 101 11 286 647 343 86 567 118 12 302 760 403 95 657 108 13 289 660 341 87 574 116 14 294 678 350 90 587 113 15 305 704 368 95 612 107 16 291 668 348 88 585 115 17 280 642 341 82 566 123 18 289 665 349 87 587 116 19 280 671 344 87 578 116 20 310 718 375 97 622 105 21 284 639 345 85 563 120 comparison 1 256 646 289 70 481 147 2 366 698 467 100 724 86 3 301 701 357 92 545 109 4 299 580 363 93 605 110 5 268 653 312 75 525 138 6 395 693 522 100 771 60 7 265 614 293 72 489 136 8 361 828 516 121 766 88 9 261 561 306 73 515 141 10 331 849 401 100 654 96 11 279 624 308 81 518 124 12 305 755 454 100 711 107 13 345 762 453 100 709 97 14 335 710 393 100 644 98

To obtain fundamental characteristics of the steel used for the crankshaft according to the present invention, the 90-millimeter round rod is hot-forged into a round rod having a diameter of 45 millimeters. The 45-millimeter round rod is cut to have a length of 250 millimeters, introduced into an atmospheric furnace, heated and held at 900 degrees Centigrade for 60 minutes. The resultant rod is cooled down to a room temperature at the cooling rate of 0.5° C./second, thereby providing the steel for an evaluation. In an ordinary crankshaft manufacturing process, the steel in a state in which the steel is hot-forged is often used. In this example, with a view of minimizing irregularities during the forging, a normalizing treatment is additionally performed. Since it is confirmed that a diameter of each of the crank pins and the crank journal of the crankshaft is within the range between 40 millimeters and 50 millimeters, the cooling rate if the steel is air-cooled after being hot-forged is within the range between 0.4 degrees Centigrade/second and 0.7 degrees Centigrade/second, a cooling rate during the normalizing treatment is controlled to fall within the same range.

Next, a hardness of a cross section of the central portion of the round rod having the diameter of 45 millimeters and the length of 250 millimeters at five points located ten millimeters under the surface layer are measured using a Vickers hardness meter. In addition, steel structures are observed at the same positions, and the area percentage of the bainite structure is calculated by an image analyzer. Since the steel used as the material for the crankshaft according to the present invention is intended to improve the fatigue strength and maintain the high machinability, a machinability of the 45-millimeter round rod is evaluated. To evaluate the machinability, a gundrill machinability of the round rod to simulate machining of the oil hole considered to be the most important in the machining of the crankshaft is evaluated. A gundrill drilling is evaluated by setting, as a machinability index, the number of drilled holes until the round rod cannot be cut since an abnormal sound is produced or a cutting tool is broken or damaged. Cutting conditions are: a cemented carbide gundrill having a diameter of six millimeters, the cutting speed of 150 m/min, a feed of 0.04 mm/rev, and a hole depth of 60 millimeters. The experimental result is shown in the Table 1.

Thereafter, hardening characteristics and the fatigue strength of the nitrocarburized steel are evaluated. Test pieces having the diameter of 15 millimeters and the length of 210 millimeters are extracted from the round rod having the diameter of 45 millimeters and the length of 250 millimeters thus obtained by machining. Further, Ono type rotating bending fatigue test pieces each having a notch formed by the machining are manufactured. The notch is formed in a central portion of each test piece so as to have a notch bottom of 1R, a notch bottom diameter of eight millimeters, and a stress concentration factor (α) of about 1.8. These Ono type rotating bending fatigue test pieces are introduced into a gas nitrocarburizing furnace applied for mass-production, and nitrocarburized at 600 degrees Centigrade for 120 minutes. For each test piece, the internal hardness of the piece before the nitrocarburizing (at a central position of a cross section of each test piece), the surface layer hardness after the nitrocarburizing, the depth of the entire hardened layer, and the internal hardness are measured by the Vickers hardness meter. For the round rod, samples for microscopic observation are produced, structures are corroded by a one-percent-by-mass Nital etching reagent, and the resultant structures are observed by a microscope. An average area percentage of the bainite structure in five visual fields of an optical microscope (a dimension of each visual field of 1.0×1.5 millimeters) is calculated by an image analyzer. The average area percentage of the bainite structure thus calculated substantially coincides with an area percentage of the bainite structure in the steel structures when a small test piece having dimensions of 10×10×70 millimeters is cut out from the central portion of the nitrocarburized round rod free from the influence of the nitrocarburizing, austenitized at 1200 degrees Centigrade for one hour, and cooled so that the cooling rate at which the sample passes through a temperature range between 900 and 300 degrees Centigrade is 0.5° C./second. Further, a repeated stress at which each test piece is not broken 10⁷th times is measured as a fatigue limit by an Ono type rotating bending fatigue tester. The result is shown in the Table 1. Namely, the crankshaft that satisfies the requirement of the present invention is not only excellent in machinability and high in fatigue strength although the surface of the steel is subjected to the nitrocarburizing treatment. 

1. A crankshaft consisting of a steel having a surface subjected to a nitrocarburizing treatment, comprising: a pin; and a journal, wherein the steel contains, as alloy elements: C having a content equal to or more than 0.10 mass % and equal to or less than 0.30 mass %, Si having a content equal to or more than 0.05 mass % and equal to or less than 0.3 mass %, Mn having a content equal to or more than 0.3 mass % and equal to or less than 1.5 mass %, Mo having a content equal to or more than 0.8 mass % and equal to or less than 2.0 mass %, Cr having a content equal to or more than 0.1 mass % and equal to or less than 1.0 mass %, and V having a content equal to or more than 0.1 mass % and equal to or less than 0.5 mass %, with a remainder consisting of Fe and inevitable impurities; the contents of the alloy elements fall within the following ranges: 2.3 mass %≦C+Mo+5V≦3.7 mass %, 2.0 mass %≦Mn+Cr+Mo≦3.0 mass %, and 2.7 mass %≦2.16 Cr+Mo+2.54V≦4.0 mass %; if a steel sample extracted from a central portion of the nitrocarburized steel free from an influence of the nitrocarburizing treatment is austenitized at 1200 degrees Centigrade for one hour, and cooled to a room temperature so that a cooling rate at which the steel sample passes through a temperature range between 900 and 300 degrees Centigrade is 0.5° C./second, then an area percentage of a bainite structure in steel structures is equal to or higher than 80 percent and a Vickers hardness measured at a cross section is equal to or higher than 260 Hv and equal to or lower than 330 Hv, and the pin and the journal have a surface hardness of a nitrocarburized layer is equal to or higher than 650 HV, a formation depth of the nitrocarburized layer is equal to or larger than 0.3 millimeters, and a hardness of the central portion is equal to or higher than 340 Hv.
 2. The crankshaft according to claim 1, wherein a content of Pb is equal to or less than 0.03 mass %.
 3. The crankshaft according to claim 1, wherein the steel contains one or more of: Nb having a content equal to or more than 0.02 mass % and equal to or less than 0.2 mass %, Ti having a content equal to or more than 0.005 mass % and equal to or less than 0.2 mass %, and Al having a content equal to or more than 0.003 mass % and equal to or less than 0.1 mass %.
 4. The crankshaft according to claim 1, wherein the steel contains one of or both of: S having a content equal to or more than 0.01 mass % and equal to or less than 0.1 mass %, and Ca having a content equal to or more than 0.0010 mass % and equal to or less than 0.010 mass %.
 5. A method for manufacturing a crankshaft according to claim 1, wherein after the steel is hot-forged or hot-forged and subjected to a solution treatment to have a shape including the pin and the journal at a temperature equal to or higher than 900 degrees Centigrade and lower than a melting point of the steel, the steel is cooled so that the cooling rate of the pin and the journal is equal to or higher than 0.3° C./second and equal to or lower than 2° C./second, whereby the area percentage of the bainite structure in the steel structures is equal to or higher than 80 percent, and thereafter, the pin and the journal are subjected to a cutting treatment, furthermore the surface of the steel is subjected to a nitrocarburizing treatment.
 6. The crankshaft according to claim 2, wherein the steel contains one or more of: Nb having a content equal to or more than 0.02 mass % and equal to or less than 0.2 mass %, Ti having a content equal to or more than 0.005 mass % and equal to or less than 0.2 mass %, and Al having a content equal to or more than 0.003 mass % and equal to or less than 0.1 mass %.
 7. The crankshaft according to claim 2, wherein the steel contains one of or both of: S having a content equal to or more than 0.01 mass % and equal to or less than 0.1 mass %, and Ca having a content equal to or more than 0.0010 mass % and equal to or less than 0.010 mass %.
 8. The crankshaft according to claim 3, wherein the steel contains one of or both of: S having a content equal to or more than 0.01 mass % and equal to or less than 0.1 mass %, and Ca having a content equal to or more than 0.0010 mass % and equal to or less than 0.010 mass %.
 9. The crankshaft according to claim 6, wherein the steel contains one of or both of: S having a content equal to or more than 0.01 mass % and equal to or less than 0.1 mass %, and Ca having a content equal to or more than 0.0010 mass % and equal to or less than 0.010 mass %.
 10. A method for manufacturing a crankshaft according to claim 2, wherein after the steel is hot-forged or hot-forged and subjected to a solution treatment to have a shape including the pin and the journal at a temperature equal to or higher than 900 degrees Centigrade and lower than a melting point of the steel, the steel is cooled so that the cooling rate of the pin and the journal is equal to or higher than 0.3° C./second and equal to or lower than 2° C./second, whereby the area percentage of the bainite structure in the steel structures is equal to or higher than 80 percent, and thereafter, the pin and the journal are subjected to a cutting treatment, furthermore the surface of the steel is subjected to a nitrocarburizing treatment.
 11. A method for manufacturing a crankshaft according to claim 3, wherein after the steel is hot-forged or hot-forged and subjected to a solution treatment to have a shape including the pin and the journal at a temperature equal to or higher than 900 degrees Centigrade and lower than a melting point of the steel, the steel is cooled so that the cooling rate of the pin and the journal is equal to or higher than 0.3° C./second and equal to or lower than 2° C./second, whereby the area percentage of the bainite structure in the steel structures is equal to or higher than 80 percent, and thereafter, the pin and the journal are subjected to a cutting treatment, furthermore the surface of the steel is subjected to a nitrocarburizing treatment.
 12. A method for manufacturing a crankshaft according to claim 6, wherein after the steel is hot-forged or hot-forged and subjected to a solution treatment to have a shape including the pin and the journal at a temperature equal to or higher than 900 degrees Centigrade and lower than a melting point of the steel, the steel is cooled so that the cooling rate of the pin and the journal is equal to or higher than 0.3° C./second and equal to or lower than 2° C./second, whereby the area percentage of the bainite structure in the steel structures is equal to or higher than 80 percent, and thereafter, the pin and the journal are subjected to a cutting treatment, furthermore the surface of the steel is subjected to a nitrocarburizing treatment.
 13. A method for manufacturing a crankshaft according to claim 4, wherein after the steel is hot-forged or hot-forged and subjected to a solution treatment to have a shape including the pin and the journal at a temperature equal to or higher than 900 degrees Centigrade and lower than a melting point of the steel, the steel is cooled so that the cooling rate of the pin and the journal is equal to or higher than 0.3° C./second and equal to or lower than 2° C./second, whereby the area percentage of the bainite structure in the steel structures is equal to or higher than 80 percent, and thereafter, the pin and the journal are subjected to a cutting treatment, furthermore the surface of the steel is subjected to a nitrocarburizing treatment.
 14. A method for manufacturing a crankshaft according to claim 7, wherein after the steel is hot-forged or hot-forged and subjected to a solution treatment to have a shape including the pin and the journal at a temperature equal to or higher than 900 degrees Centigrade and lower than a melting point of the steel, the steel is cooled so that the cooling rate of the pin and the journal is equal to or higher than 0.3° C./second and equal to or lower than 2° C./second, whereby the area percentage of the bainite structure in the steel structures is equal to or higher than 80 percent, and thereafter, the pin and the journal are subjected to a cutting treatment, furthermore the surface of the steel is subjected to a nitrocarburizing treatment.
 15. A method for manufacturing a crankshaft according to claim 8, wherein after the steel is hot-forged or hot-forged and subjected to a solution treatment to have a shape including the pin and the journal at a temperature equal to or higher than 900 degrees Centigrade and lower than a melting point of the steel, the steel is cooled so that the cooling rate of the pin and the journal is equal to or higher than 0.3° C./second and equal to or lower than 2° C./second, whereby the area percentage of the bainite structure in the steel structures is equal to or higher than 80 percent, and thereafter, the pin and the journal are subjected to a cutting treatment, furthermore the surface of the steel is subjected to a nitrocarburizing treatment.
 16. A method for manufacturing a crankshaft according to claim 9, wherein after the steel is hot-forged or hot-forged and subjected to a solution treatment to have a shape including the pin and the journal at a temperature equal to or higher than 900 degrees Centigrade and lower than a melting point of the steel, the steel is cooled so that the cooling rate of the pin and the journal is equal to or higher than 0.3° C./second and equal to or lower than 2° C./second, whereby the area percentage of the bainite structure in the steel structures is equal to or higher than 80 percent, and thereafter, the pin and the journal are subjected to a cutting treatment, furthermore the surface of the steel is subjected to a nitrocarburizing treatment. 